Pulmonary surfactant lowers the surface tension arising at the air-liquid interface of the internal alveolar wall, thus preventing the lungs from collapsing at the end of expiration. Surfactant deficiency is a dysfunction which commonly affects preterm infants and causes RDS, a disease which can be effectively treated with natural surfactants extracted from animal lungs. The main constituents of these surfactant preparations are phospholipids, such as 1,2-dipalmitoyl-sn-glycero-3-phosphocholine commonly known as di-palmitoyl-phosphatidyl-choline (DPPC), phosphatidylglycerol (PG) and surfactant hydrophobic proteins B and C (SP-B and SP-C). The hydrophilic surfactant proteins SP-A and SP-D, which are C-type (Ca2+-dependent) collagenous lectins and thought to act primarily in the host-defence system, are normally not included in the surfactant preparations due to the organic solvent extraction procedures employed. Modified natural surfactant preparations obtained from animal tissues are used in current therapeutical practice. These preparations usually consist of the aforementioned components with the exception of hydrophilic proteins, which are removed upon extraction with organic solvents.
Owing to the drawbacks of the surfactant preparations from animal tissues, such as the complexity of the manufacturing and sterilization processes and possible induction of immune reactions, attempts to prepare artificial surfactants have been made.
In the strict sense of the word, artificial surfactants are mixtures of phospholipids only or mixtures of phospholipids and other synthetic lipids. Reconstituted surfactants are artificial surfactants added with hydrophobic proteins—either isolated from animal tissues or obtained through recombinant techniques—or synthetic peptidic derivatives of such proteins.
The properties and the activity of reconstituted surfactants greatly depend not only on the protein/peptide components, but also on the composition of the phospholipid mixture.